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Prop protection

Jun 24, 2010

We’ve all had that experience, sailing peacefully along on a sunny afternoon in coastal waters, an eye on the sails, thoughts drifting off, perhaps toward the evening’s hoped-for destination, when suddenly — clunk! — your headway drops to nothing and the boat veers awkwardly like a fish caught by the tail. You’ve snagged a lobster or crab pot in your propeller. You then execute a series of futile turns and sail combinations in a feeble, hopeless attempt to free the wheel, knowing in your heart that the only thing that will clear the line involves you, a knife clenched between your teeth and getting wet and banged up under the rolling hull. You know the line is a snarled mess around the blades and shaft, extending almost bar-tight, out of sight below you to the sea floor.

What’s even worse is when you’re motoring along and a snagged line violently causes the motor to stall; the line has become so jammed on the shaft that it has welded itself together, and it might be hours before you can continue on your course.

I’ve fouled a propeller in pot warp or mooring lines more times than I can remember — both in the open sea (fishing net) and near the coast (lobster or crab gear). And although the experiences run together, I can clearly remember the awkward and dangerous experience that hacking at a line underwater involves.

Sometimes there’s no avoiding a snagged propeller, but this story examines several methods for avoiding snags and then, should they occur, what you can do to prepare for such eventualities and keep yourself and your boat safe.

Clearly, some boat designs seem to snag any lobster pot within 30 feet of the boat, almost as if they suck them toward the hull and then into the propeller. Other boats seem impervious to catching traps — no matter how many times you see the lobster pot slip beneath the boat and you just know you’ve snagged one, just as quickly it pops up in your wake and slips harmlessly past.

Cutting blades
An obvious answer to the potential of a fouled propeller is to install cutting blades on the shaft and wheel. The most visible brand, Spurs, has been installed on more than 100,000 boats around the world, from small yachts to full-size ships. Shaft sizes for shaft-mounted cutters range from half an inch to seven inches. For prop-mounted blades, the size of the shaft is potentially unlimited — such that they are installed on vessels over 1,000 feet long with shafts over two feet in diameter.

The Spurs system consists of two rotating blades that are clamped to the shaft. A fixed blade is held in place by a V-block which is fastened to the bearing housing. All of the blades are double-sided so that they will cut whether the engine is running ahead or astern, and for both left-hand or right-hand turning propellers, according to Susan Correa of Spurs Marine, which is based in Fort Lauderdale, Fla. Sound-dampening “plugs” between the stationary cutter and the wedge holding block prevents metal-to-metal contact.

Even at slow speeds — when, for example, a vessel is making way under sail — the cutters still slice through line because of the weight and inertia of the spinning blades. “We’ve had no trouble cutting lines on sailboats,” Correa said. “Our cutter cuts with minimal resistance with the guillotine effect, therefore even in a slow speed the cutters cut any obstructions.”
Skip Strong, a Penobscot Bay and River Pilot based in Searsport, Maine, has a Shaft Razor installed on the company’s pilot boat — a product made by Evolution Marine in Rockland, Maine, where the vessel is berthed.

“We still have to have a diver go down every once in a while, but not as much as in the past (without cutters),” Strong said. “We usually run with a search light on at night to help us avoid the gear.” He said that roughly 10 percent of the vessels — from tugs to tankers to cruise ships — that call on the Maine midcoast ports he and his partners serve are equipped with cutting blades. Most vessels build the cost of divers into their operations budgets, sending divers down regularly to clear fouled line. Even when a propeller is spinning (its blades having cut through the line), line can nonetheless tangle in the blades and cause friction, sacrificing fuel efficiency.

“We usually send a diver down when the temperature gauge begins to creep up,” said a ferry operations manager in the Northeast who preferred not to be identified for this story because of potential conflicts with local fishermen whose gear the company’s vessels routinely snag.

But what about when a Kevlar jib sheet is hanging overboard and it gets sucked into the propeller’s cutting blades?

“Whatever rope cutter design a vessel uses there will be circumstances where the result is not as expected,” Alan Carr of Evolution Marine said. “For instance, no matter what the brand or design, if the rope is held perpendicular to the shaft, entanglement is guaranteed. Other non-functioning instances would involve entanglement with wire-cored rope or mooring chains.”

Evolution’s Shaft Razor Cutter serves the fishing, commercial, and recreational market — about 40 percent are installed on commercial vessels, he said. Initially, the workboat market was leery of either promoting or installing cutting gear on commercial vessels, since doing so would seem to be promoting the destruction of underwater lines.

“If a vessel were to become entangled in trap gear, cutting the buoy at the GPS location affords some retrieval possibilities rather than having the gear tow where it cannot be found,” Carr said. “Additionally, some of our commercial customers have opted for the product after (fouling their propellers and needing a tow).”

The Shaft Razor features a pair of round, serrated blades, one perpendicular, and the other parallel, to the shaft and at 90 degrees to one another. This configuration makes the cutter particularly “aggressive” at cutting through lines, regardless of the angle of the approaching line. While having the cutters does not guarantee tangle-free sailing, Carr said the combination also limits shaft and alignment damage in the event of a really nasty snag, such as what might occur from contact with metal or high-strength line.

The basic difference between Spurs and the Shaft Razor, according to the manufacturers, is that Spurs cut suddenly and instantly like a guillotine, while the Shaft Razor saws away at the line because of its circular shape.

“With the Shaft Razor, one may feel the vessel slow hesitantly and then clear,” Carr said. The units are machined several thousandths of an inch over the shaft diameter as a “slid fit,” Carr said. The shaft is then dimpled (drilled) to catch the head of the stainless cone head set screws. Loctite or silicone is utilized on the set-screws in the thread cavity to prevent marine growth build-up.

Maintenance of the Shaft Razor is fairly simple, provided the vessel is on the hard.

“Check the points of the serrations when the vessel happens to be out of the water,” Carr said. “If one or two are bent over, merely straighten with a small bronze hammer. Occasionally, take a half-round file and dress the serration edges. This can be easily done with a small hobby grinder. Check that the set-screws are tight to the shaft. All of this can be done with the unit still on the shaft.”

Dealing with a snag
The whole strategy behind the marketing of these cutting units is built around the belief that the blades will cut through the majority of what it comes in contact with. Needless to say, if the cutting blades fail to cut whatever is snagged, whether a Kevlar sheet, an enormous cluster of line, or a line laced with metal, you have an even bigger problem on your hands when it comes time to clear the propeller — because now you have a set of sharp, serrated blades on the shaft that would just as soon cut through a finger as a piece of pot warp.

“Have you ever been on a boat with cutters that have picked up a line? I have, and don’t want to be again. It was on the close side of violent,” Rob Benson, service manager at Portland Yacht Services in Portland, Maine, said. “Furthermore, I can’t imagine having that happen 10 times a year would do anything positive for your alignment.”

“I consider protection for the prop to be more directly related to safety as a result of lines on the boat going overboard maneuvering in the harbor or at sea,” Phin Sprague, owner of Portland Yacht Services and of the 65-foot Alden-designed staysail schooner called Lion’s Whelp, added. “We have these new Kevlar lines for sheets and I am concerned that when we are handling sails in Gulf Stream squalls and have the boat essentially stopped, a line will get washed overboard and into the prop. Unless you have been in one of the night squalls where visibility is zero in rain and spray and an inexperienced helms person heads up instead of off, and the whole boat is a mass of loose lines and thrashing sails, you probably wouldn’t appreciate the number of lines that can go overboard and potentially get into the prop.”

Diving equipment
Sooner or later, then, you will have to go over the side to free a tangled prop. Hopefully, it’s not at night in the conditions described above. A number of years ago I wrote a piece for this magazine that compared products for underwater breathing equipment (see Emergency dive gear, Ocean Navigator Sept/Oct 1998, issue #92), from portable compressors to emergency scuba gear. Whatever the preference, it is wise to have some combination of the following equipment: a good diving mask; a portable scuba tank like Spare Air or conventional dive gear; a hookah-style breathing tube connected to an air compressor on the boat; a sharp, preferably serrated knife with a fixed blade that won’t collapse on your fingers; a pair of swimming fins; a pair of Neoprene gloves to protect your hands from burrs on the prop blades; and a wetsuit for cold water.

When diving in cold Maine waters, I wore a Neoprene hood when working underwater, primarily for warmth. But after getting bonked on the head the first 10 times, I realized the foam acted as a helmet, too, and now I carry a Neoprene diving hood for going over the side, regardless of the temperature of the water.
 
Once, when sailing off Nantucket on a 48-foot cutter on a delivery from Florida to Maine, the boat I was on snagged a section of fishing net that turned out to be about the size of a blanket. We were motoring at the time, and the thunk of the engine, followed by an immediate stall, left no question as to what happened. But the preparedness of the skipper — who happened to be Ocean Navigator Contributing Editor Chuck Husick — was a pleasure to witness: Chuck immediately went below and locked the shaft from inside the engine room; fired up the generator; pulled his well-oiled air compressor from a locker and plugged it into a pilothouse electrical outlet; donned a full wetsuit, mask, snorkel, and fins; lashed a serrated knife to his thigh; rigged the overboard ladder; and then disappeared over the side with the hookah in his mouth as dashingly as a Jules Verne character. He reappeared a few minutes later with the offending net in his grip. We were stopped for all of 10 minutes. (I don’t want to describe the times when I was less prepared — bloody and painful experiences.)

One final note bears mentioning: whenever I’ve cut a line I’ve made every attempt to rejoin the buoy to the fishing or mooring gear before setting it loose. A string of lobster gear can cost many hundreds of dollars that would be lost otherwise. A little goodwill goes a long way.

Cutting blades require little maintenance and are not particularly expensive as far as boat gear goes — a few hundred dollars plus installation. Whatever the choice, it pays to think ahead about how to deal with these underwater nuisances.

Twain Braden is director of Camp Glen Brook in Marlborough, N.H. His upcoming book, The Ultimate guide to Sailing & Seamanship, will be published by Skyhorse in December.

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